This invention relates to prosthetic implants for skeletal replacement, reconstruction and attachment in humans and animals, and, more particularly, to the design and method of installation of such devices that would reduce their loosening with time.
Implantable devices are used to partially or completely replace joints or bone segments in humans and animals, or to provide direct skeletal attachment of external prostheses to the residuum.
The known approaches to attaching the implants include fitting the implant into the medullary canal of the bone by force; securing the implant in the bone with screws or pins; bonding the implant to the bone with various adhesives; use of porous structures to stimulate ingrowth of the bone into the implant's surface.
One of the major problems involved in the use of such devices is the loosening of the attachment between the prosthetic implant and the associated bone. Loosening occurs mainly due to the cyclical application of bending moments during locomotion which eventually destroy the bond between the implant and the bone [3-5].
To decrease loosening, a more precise installation technique, anchoring elements, and surgical assemblies were introduced in U.S. Pat. Nos. 5,702,445 (6], 6,159,216 [7], 6,520,966 [8], and 7,001,394 [9]. Another approach was introduced in U.S. Pat. No. 4,828,566 where a recess is carved from the implanted region of the prosthesis in the proximal medial region, and a U-shaped wire mesh structure is fitted within the recess. The wire mesh structure allows for an ingrowth of bone tissue in the medial narrow side of the shank and for the absorption of shear micro movements between the bone and the implant [10].
One of the reasons for loosening, after all these approaches or their several combinations are used, is that the approaches are all in conflict with the structure and function of the medullary cavity canal, into which the implants are inserted. With the conventional method of installation of the shank, a drill first bores the tube bone to prepare an area into which the shaft of the implant fits exactly. Then, the implant is installed into the bone as described in: http://biomedtrix.com/sur.html.
The procedure destroys, completely or in part, the layer of endosteal bone trabeculae, or endosteum, which fills the medullary cavity of the bone [11], as illustrated in FIGS. 1 and 2.
After the medullary canal is drilled (see FIG. 2) in preparation for device implantation, osteocytes begin to remodel the internal canal walls and fill the gaps between the implant and the walls, including the specially designed cavities or pores in the implant. The remodeling proceeds in the direction out from the outer walls toward the interior walls of the medullary canal [12].
Such ossification fixes the implant inside the bone canal by developing multiple micro locks, and is therefore useful for anchoring and preventing further loosening. However, the pre-existing position of the endosteum limits the potential volume of the remodeled ossified bone tissue in the outward-inward direction. This is the natural mechanism which protects the area designated for bone marrow from filling with cortical bone, in the process of bone remodeling as a consequence of bone fracture [12].
Therefore for additional anchoring, to augment the effects of slight ossification, the implant is often secured with screws (1) (see FIG. 3) inserted from the outside of the bone into the implanted shaft of the prosthesis, as described in:
http://www.engr.ncsu.edu/news/newsletters/pdfs/frontline 1105.pdf.
This locking and anchoring approach requires additional operation time and techniques for exact positioning of the screws relative to the holes of the shaft implanted into the medullary canal.
All described approaches depend on ossification inside the medullary canal, which has aforementioned natural limitations to the volume of the remodeled bone tissues.
In contrast to ossification in the outer-to-inner direction, the ossification in the direction of the longitudinal axis of a bone can be achieved in significantly higher volumes of remodeled bone tissues. This well-known phenomenon is utilized in bone lengthening techniques, when an external apparatus is applied for the fixation of the bone fragments that are created after the bone is dissected perpendicularly to its longitudinal axis. Then, with the aid of the given external apparatus, bone fragments are moved apart 1-2 mm per day. Continued ossification, when properly controlled, allows the bone to lengthen up to 33% of its original length [13, 14]. Similar volume of ossification occurs in the lateral direction when the bone is widened [15]. However, this approach has never been applied to lock the implanted shaft.
Accordingly, it is an object of the present invention to prevent the occurrence of the implant loosening. It is another object of the invention to utilize the natural anchoring (osseolocking) of an implant in a bone by introducing a corresponding device and method of its installation.